1. Field of the Invention
Example embodiments generally relate to a fixing device, an image forming apparatus, and an image fixing method, and more particularly, to a fixing device, an image forming apparatus including the fixing device, and an image fixing method for fixing a toner image on a recording medium.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium (e.g., a sheet) according to image data using electrophotography. Thus, for example, a charger uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner particles to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a sheet or is indirectly transferred from the image carrier onto a sheet via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the sheet; finally, a fixing device applies heat and pressure to the sheet bearing the toner image to fix the toner image on the sheet, thus forming the image on the sheet.
In such image forming apparatuses, oil is applied to a fixing member of the fixing device, which contacts the toner image on the sheet, to separate toner particles forming the toner image from the fixing member and to maintain fixing property, or the ability of the fixing member to fix the toner image on the sheet. Understanding the way in which the oil is applied requires a detailed discussion of the structure of a typical conventional fixing device.
In order to shorten a warm-up time period of the fixing device, an endless fixing belt is often used as the fixing member. FIG. 1 is a sectional view of a fixing device 100RA including just such an endless fixing belt.
As can be seen in FIG. 1, the fixing device 100RA loops an endless fixing belt 92 around a fixing roller 84 and a heating roller 83 in such a manner that the fixing roller 84 and the heating roller 83 apply a reference tension to the fixing belt 92. A pressing roller 85 rotates and presses against the fixing belt 92 and the fixing roller 84 to form a fixing nip portion AR between the fixing belt 92 and the pressing roller 85. A heater 86 is disposed inside the heating roller 83, and heats the fixing belt 92 via the heating roller 83. Similarly, a second, separate heater 87 is disposed inside the pressing roller 85, and heats the pressing roller 85. The fixing belt 92 and the pressing roller 85 apply heat and pressure to a sheet bearing a toner image at the fixing nip portion AR to fix the toner image on the sheet.
An oil application roller R1 serves as an oil applier for applying oil to the fixing belt 92. For example, the rotating oil application roller R1 applies oil supplied from an oil supplier to the fixing belt 92 at a contact point 95 at which the oil application roller R1 contacts the fixing belt 92. The rotating fixing belt 92 moves an oiled portion of the fixing belt 92 to which oil is applied to the fixing nip portion AR to separate the sheet bearing the toner image from the fixing belt 92. At the fixing nip portion AR, a part of the oil on the fixing belt 92 moves from the fixing belt 92 onto a surface of the pressing roller 85 to separate the sheet bearing the toner image from the pressing roller 85. Surplus oil carried on the pressing roller 85 then flows into an oil pan 96 provided under the pressing roller 85, and is collected by the oil pan 96.
However, not enough oil may adhere to the pressing roller 85, and consequently a sheet bearing a fixed toner image may not separate cleanly from the pressing roller 85. For example, when a plurality of sheets continuously passes through the fixing nip portion AR, oil can only move from the fixing belt 92 onto the pressing roller 85 during an interval between successive sheets, resulting in a shortage of oil.
To prevent or reduce such shortage of oil on the pressing roller 85, another oil applier may be provided for the pressing roller 85 to apply oil to the pressing roller 85. However, the oil thus applied to the pressing roller 85 may adhere to a foremost sheet fed into the fixing device 100RA immediately after the fixing device 100RA is driven.
On the other hand, gravity can easily cause a sheet passing through the fixing nip portion AR to adhere to or wrap around the pressing roller 85 at an exit side of the fixing nip portion AR.
To address these problems, another related-art fixing device 100RB includes an oil applier R3 for applying oil to the pressing roller 85 by directly contacting the pressing roller 85, as illustrated in FIG. 2. However, the oil applier R3 may not apply an amount of oil sufficient to form a toner image at high linear speed or to separate coated paper, which absorbs oil easily, from the pressing roller 85. Moreover, it is difficult to locate an oil applier capable of applying a substantial amount of oil to the pressing roller 85 at a position above the pressing roller 85 due to limited space.